As a process for the gasification of coals, there have been hitherto practically employed a fixed bed gasification process, a fluidized bed process, an entrained bed process and the like process, wherein air or oxygen is used as a gasification agent. A gas obtained by these gasification processes has a heating value of 1,000-4,000 Kcal/m.sup.3. In order to produce a gas having a heating value near that of natural gasses (6,000-9,900 Kcal/m.sup.3) by the gasification of coals, it is necessary to use hydrogen as a gasification agent. Since reaction of coals with hydrogen greatly differs from that with oxygen, however, it is impossible to obtain satisfactory results when the conventional process in which air or oxygen is to be used as a gasification agent is adopted as is.
The present inventors have made various studies with a view to developing a process in which the hydrogenolytic gasification, under pressure, of coals can occur with a good efficiency. To collect basic data concerning hydrogenolytic gasification reactions, under pressure, of coals, the present inventors have first carried out reactions of a coal with hydrogen by passing a pressurized hydrogen gas, under various reaction conditions, through a reaction tube packed with a coal powder while externally heating the tube by means of an electric furnace. The results of the experiments under various pressures are shown in Table 1.
TABLE 1 ______________________________________ Char obtained by carbonizing Taiheiyo coal* at 500.degree. C. (particle Raw material size: 30-60 mesh) ______________________________________ Heat-up rate (.degree.C./min) 2 Reaction temperature (.degree.C.) 750 Retention time at the reaction temperature(hr) 3 Hydrogen feed rate(l/hr) 500 Pressure(Kg/cm.sup.2) 1 10 20 30 40 70 Gross heating value of gas product(Kcal/m.sup.3) 3700 4510 5505 5985 6029 9206 Conversion rate of raw material(%) 21.5 30.3 39.2 41.7 51.9 60.0 ______________________________________ *non-caking bituminous coal
It will be seen from Table 1 that both the gross heating value of the gas product and the conversion rate of the raw material increase with the rise of the reaction pressure and, hence, a relatively large amount of a gas having the high heating value near that of natural gases is obtainable when the hydrogenolytic gasification is conducted under a pressure of about 70 Kg/cm.sup.2.
Next, the change in temperature in the coal layer with the lapse of time and the change in amount of methane generated with the lapse of time were investigated in association. The results are graphically shown in FIGS. 1 and 2. In each of these graphs, the axis of abscissa represents time lapse of the experiment (hr), while the left and right axes of ordinate stand for amount of methane generated (l/hr) and temperature (.degree.C.), respectively. In the graphs, the line A shows the change in temperature within the coal layer with the lapse of time while the line B shows the change in amount of methane generated with the lapse of time.
The results, in the case of 10 Kg/cm.sup.2 of reaction pressure, shown in FIG. 1 indicate that the amount of methane generated significantly increases at a temperature in the coal layer of 650.degree. C. and more, and reaches its maximum at 750.degree. C. But, the gross heating value of the gas product is small as appreciated from the experiment results shown in Table 1. And, in the reaction of this case, the temperature in the coal layer increases proportionally with the increase in the outside temperature of the reaction tube, showing that no great generation of heat due to the gasification reaction occurs.
In the results, shown in FIG. 2, in the case of 70 Kg/cm.sup.2 of reaction pressure, two peaks are present, with respect to the amount of methane generated, in a lower temperature side (approximately 550.degree. C.) and a higher temperature side (approximately 750.degree. C.). These two peaks appear when the reaction is performed under a pressure of 30 Kg/cm.sup.2 or more. When the lower temperature side peak of the amount of methane generated appears, the temperature in the coal layer increases instantaneously from about 400.degree. C. to 800.degree. C., showing the occurrence of an abrupt exothermic reaction.
From the foregoing results of the basic reaction experiments, it may be concluded that in order to obtain a gas having such a high heating value as that of natural gases, it is necessary to allow such the hydrogenolytic gasification reaction of coals as represented in FIG. 2 by the lower temperature side peak with respect to the amount of methane generated to efficiently occur so as to obtain a gas containing methane as its major component and having a high heating value, while utilizing the abrupt exothermic phenomenon, which occurs simultaneously therewith, for maintaining the temperature inside a reaction furnace in a range suitable for the gasification.